1. Field of the Invention
The invention relates to methods for fabricating semiconductor structure, in particular, methods for forming at least two different nanowire structures with different diameters on one substrate.
2. Description of the Prior Art
The fabrication of a nanowire field effect transistor (FET) with a gate dielectric and a gate conductor surrounding the nanowire channel (also known as a gate-all-around nanowire FET) includes suspension of the nanowires. Suspension of the nanowires allows for the gate conductor to cover all surfaces of the nanowires.
The fabrication of a gate-all-around nanowire FET typically includes the following steps: (1) Definition of the nanowires between source and drain regions by patterning a silicon-on-insulator (SOI) layer. (2) Suspension of the nanowires by isotropic etching that undercuts the insulator on which the nanowires are resting. This etching step also undercuts the insulator at the edge of the source and drain region. The overhang/undercut that forms may not be a desirable outcome. (3) A blanket and conformal deposition of the gate dielectric and the gate conductor. The gate dielectric and the gate conductor wraps around the suspended nanowires and fills the undercut at the edge of the source and drain regions. (4) Definition of the gate line which includes the etching of the gate line and removal of the gate dielectric and the gate conductor material from all regions outside the gate line, including gate material deposited in the cavities at the edge of the source and drain regions.